The conventionally known methods for producing acetic acid on an industrial scale include the fermentation method; the method of carbonylation of methanol by a reaction in a homogeneous liquid phase system with the use of rhodium and iodine as a catalyst, the method of oxidation of a hydrocarbon (butane, naphtha, etc.) by a reaction in a heterogeneous solid phase system with the use of an organic-soluble salt catalyst (manganese naphthenate, cobalt naphthenate, nickel naphthenate, etc.); the ethylene two-step oxidation method which comprises oxidizing ethylene to thereby once form acetaldehyde and then oxidizing the resulting acetaldehyde in a homogeneous liquid phase system with the use of manganese acetate or a mixture of manganese acetate, copper acetate and cobalt acetate to thereby give acetic acid; and a method which comprises reacting ethylene with oxygen in a gas phase with the use of metallic palladium and heteropolyphosphoric acid as the main catalyst (JP-A-7-89896; the term "JP-A" as used herein means an "unexamined published Japanese patent application").
In each of these methods, acetic acid is obtained in the form of an aqueous solution. To obtain dehydrated and purified acetic acid, it is therefore needed to remove water from this aqueous solution by a method as inexpensive as possible.
Distillation is generally employed in order to industrially obtain purified acetic acid from an aqueous solution of acetic acid. To separate water from acetic acid by a conventional distillation method, however, it is needed to use a distillation column provided with a large number of plates (i.e., 70 or more) since the boiling point of acetic acid (117.8.degree. C. under atmospheric pressure) is close to that of water. In addition, a large amount of water having a large heat of vaporization should be distilled off from the column top, which requires a large-scaled equipment and much energy. Due to the low relative volatility of water to acetic acid, furthermore, it is needed to set a large reflux rate at the column top, which lowers the efficiency of the process.
Various proposals have been made to solve this problem. For example, there has been known a method which comprises subjecting an aqueous solution of acetic acid (hereinafter referred to as the "feedstock solution") to azeotropic distillation together with an azeotropic agent capable of forming an azeotrope with water and thus distilling off the minimum azeotrope of water and the azeotropic agent from the column top while recovering the acetic acid thus concentrated from the column bottom (JP-B-43-16965, JP-B-61-31091, etc.; the term "JP-B" as used herein means an "examined Japanese patent publication"). Although this method is advantageous in that the reflux rate at the column top can be lowered and thus the energy required for the distillation of water can be reduced, a large amount of water should be distilled off from the column top similar to the conventional distillation methods. Thus no sufficient effect of saving energy can be achieved thereby.
As a method other than the azeotropic distillation method, there has been known the extraction method. This method generally comprises making a water-insoluble organic solvent, which is employed as the extracting medium, in contact with the feedstock solution, thus extracting acetic acid into the extracting medium phase and then separating and purifying the acetic acid from the extracted solution by, for example, distillation. An important factor of this extraction method resides in the selection of an appropriate extracting medium which has a small partition coefficient with water and allows sufficient dissolution of acetic acid therein.
Regarding the selection of an appropriate solvent, a number of proposals have been made to employ a solvent which has a boiling point higher than that of acetic acid and allows sufficient dissolution of acetic acid therein, since a solvent with a higher boiling point generally has the smaller partition coefficient with water. In JP-A-60-25949 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), for example, acetic acid is extracted from a feedstock solution with the use of a high-boiling solvent comprising a C.sub.7 aliphatic ketone as the major component and, after stripping the water contained in the extracted solution, acetic acid is separated from the high-boiling solvent by distillation. In JP-B-59-35373, extraction is performed by using a tertiary amine, which has a boiling point higher than that of acetic acid, together with an oxygen-containing organic solvent, which also has a boiling point higher than that of acetic acid, and the extracted solution is dehydrated by distillation followed by the distillation of the dehydrated mixture again to thereby give the acetic acid. In JP-B-60-16410, a specific secondary amide is employed as an extracting medium and acetic acid is separated from the extracted solution by distillation. Furthermore, U.S. Pat. No. 4,143,066 proposes to use trioctylphosphine oxide as a high-boiling solvent capable of selectively extracting acetic acid.
There have been also known methods wherein a mixture of a low-boiling solvent with a high-boiling solvent is employed as an extracting medium. In JP-B-1-38095, for example, a solvent mixture comprising ethyl acetate with diisobutyl ketone is used. Further, U.S. Pat. No. 2,175,879 discloses a method wherein extraction and azeotropic distillation are carried out at the same time. In this method, a feedstock solution is divided into two portions and one portion is extracted with a low-boiling solvent while another portion is subjected to azeotropic distillation with the use of an azeotropic agent such as butyl acetate. By the multipurpose use of the heat of condensation of the gas at the azeotropic distillation column top, the low-boiling solvent in the extracted solution is recovered from acetic acid via distillation, thus saving energy.
When a high-boiling solvent is used as an extracting medium in the extraction methods or the extraction/azeotropic distillation methods as described above, the amount of water taken up into the extracting medium phase is generally reduced but the partition coefficient thereof with acetic acid is also lowered. As a result, the extracting medium should be used in a large amount and, in its turn, the scale of the equipment is enlarged. In the subsequent step of the separation of acetic acid from the extracting medium by distillation, moreover, it is needed to distill off acetic acid having a relatively large latent heat of evaporation from the column top, which brings about an increase in the energy cost. When this separation is performed via the minimum azeotropic distillation with water, the extracting medium has a boiling point higher than that of acetic acid and thus its minimum azeotropic distillation temperature is close to the boiling point of acetic acid. Accordingly, it is difficult to obtain highly pure acetic acid at a high yield in this case.
The method, wherein a mixture comprising a low-boiling solvent together with a high-boiling solvent is used as the extracting medium, is more beneficial than the method with the use of a low-boiling solvent alone. In the former case, however, a large amount of water is taken up into the extracting medium phase, which enlarges the load in the azeotropic distillation. In this case, it is also required to separate the high-boiling solvent form acetic acid by distillation. Thus it is not always beneficial from the viewpoint of energy consumption.
The present invention has been completed in order to solve the above-mentioned problems.